An apparatus and method to position a wafer onto a wafer holder and to maintain a uniform wafer temperature is disclosed. The wafer holder or susceptor comprises a recess or pocket whose surface is concave and includes a grid containing a plurality of grid grooves separating protrusions. The concavity and grid grooves define an enclosed flow volume between a supported wafer and the susceptor surface, as well as an escape area, or total cross-sectional area of the grid grooves opening out from under the periphery of the wafer. These are chosen to reduce the wafer slide and curl during wafer drop-off and wafer stick during wafer pick-up, while improving thermal uniformity and reducing particle problems. In another embodiment, centering locators in the form of thin, radially placed protrusions are provided around the edge of the susceptor pocket to reduce further the possibility of contact between the wafer and the outer edge of the susceptor. These features help to achieve temperature uniformity, and therefore quality of the process result, across the wafer during processing.
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1. A method of maintaining process temperature at an outer periphery of a substrate positioned in a recess on a surface of a substrate holder, the method comprising providing a plurality of centering locators spaced around an outer edge of the recess and located to center the substrate within the recess, each centering locator having an inner surface which is spaced radially inward from an annular shoulder that defines the outer edge of the recess, providing each of the centering locators with a circumferentially thin dimension so that the outer periphery of the substrate engaging the centering locator has no direct contact with the shoulder.
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The present application is a divisional of and claims the priority benefit under 35 U.S.C. § 121 from U.S. application Ser. No. 10/455,267 filed on Jun. 4, 2003 now U.S. Pat. No. 6,729,875, which is a divisional of U.S. application Ser. No. 09/747,173 filed on Dec. 22, 2000, issued as U.S. Pat. No. 6,634,882, entitled “Susceptor Pocket Profile to Improve Process Performance,” the disclosure of which is incorporated herein by reference.
The invention relates generally to susceptors for supporting semiconductor substrates in process chambers, and, more particularly, to configuring wafer support surfaces to maintain uniform substrate temperature.
Semiconductor fabrication processes are typically conducted with the substrate supported within a chamber under controlled conditions. For many processes, semiconductor substrates (e.g., silicon wafers) are heated inside the process chamber. For example, substrates can be heated by direct physical contact with a heated wafer holder and/or by radiation from a radiant heating source. “Susceptors,” for example, are wafer supports that absorb radiant heat and transmit absorbed heat to the substrate.
In a typical process, a reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of reactant material on the wafer. Through sequential processing, multiple layers are made into integrated circuits. Other exemplary processes include sputter deposition, photolithography, dry etching, plasma processing, and high temperature annealing. Many of these processes require high temperatures and can be performed in similar reaction chambers.
Various process parameters must be controlled carefully to ensure high quality in deposited films. One critical parameter is the temperature of the wafer during the processing. During CVD, for example, there is a characteristic temperature range in which the process gases react most efficiently for depositing a thin film onto the wafer. Temperature control is especially critical at temperatures below the mass transport regime, such as silicon CVD between about 500 C and 900 C (kinetic regime, about 500 C to 900 C for silicon CVD using silane). In this kinetic regime, if the temperature is not uniform across the surface of the wafer, the deposited film thickness will be uneven.
In recent years, single-wafer processing of large diameter wafers has become more widely used for a variety of reasons, including the need for greater precision in process control than can be achieved with batch-processing. Typical wafers are made of silicon, most commonly with a diameter of about 150-mm (6 inches) or of about 200-mm (8 inches) and with a thickness of about 0.725 mm. Recently, larger silicon wafers with a diameter of about 300 mm (12 inches) and a thickness of about 0.775 mm have been introduced, as they exploit, even more efficiently, the benefits of single-wafer processing. Even larger wafers are expected in the future.
The inventor has found many quality control problems affecting yield result from handling issues associated with susceptors, including substrate slide, stick and curl. These occur during placement and subsequent removal of substrates in high temperature process chambers.
Slide occurs during drop off when a cushion of gas in the susceptor recess or pocket is unable to escape fast enough to allow the substrate to fall immediately onto the susceptor. The substrate floats momentarily above the susceptor as the gas slowly escapes, and it tends to slide off center. Thus, the substrate may not rest in the center of the pocket where it was intended, and uneven heating of the substrate may result. Sliding to the edge of a susceptor pocket causes local cooling where the substrate is in contact with the pocket edge and results in poor thickness uniformity, poor resistivity uniformity and crystallographic slip, depending on the nature of the layer being deposited. These non-uniformities, due to inconsistencies in the wafer drop position, greatly increase the difficulty in optimal tuning of the process. Similarly, non-uniformities in temperature can cause non-uniformities in etch, anneal, doping, oxidation, nitridation and other fabrication processes.
Conversely, during pickup, stick occurs when the substrate clings to the underlying support because gas is slow to flow into the small space between the wafer and the surface of the pocket. This creates a vacuum effect between the substrate and the support as the substrate is lifted. Stick is a potential contributor to particle contamination and, in extreme cases, has caused lifting of the substrate holder on the order of 1 to 2 mm.
Curl is warping of the substrate caused by a combination of both radial and axial temperature gradients in the substrate. Severe curl can cause the substrate to contact the bottom side of a Bernoulli wand, and can similarly affect interaction with other robot end effectors. In the case of a Bernoulli wand, the top side of the substrate can scratch the Bernoulli wand and cause particulate contamination, significantly reducing yield. The design and function of a Bernoulli wand are described in U.S. Pat. No. 5,997,588 and are included by reference herein.
In
Cooling at the wafer edge renders the temperature of the wafer non-uniform. Given that thin film deposition rates (and many other fabrication processes) are strongly temperature dependent, especially for CVD in the kinetic regime, film thickness, and resistivity, will be non-uniform across a wafer processed under conditions of temperature non-uniformity. Consequently, there is a need for an improved substrate support that facilitates substrate pick-up and drop-off while promoting temperature uniformity.
In satisfaction of this need and in accordance with one aspect of the invention, a substrate support is provided with a grid of grooves extending into a concave surface, which can hold a generally flat substrate, such as a silicon wafer, for processing. The concavity and grooves are configured to minimize stick, slide and curl, while still maintaining desirable thermal properties. Methods for configuring the support and for supporting a substrate thereon are also provided.
In accordance with another aspect of the invention, centering locators, arranged radially along an inside edge of an annular shoulder of a substrate holder, are supplied. The centering locators establish a distance between the substrate and the annular shoulder to prevent direct thermal contact therebetween.
Further features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follow, when considered together with the attached drawings, which are intended to illustrate and not to limit the invention.
As noted in the Background and Summary section above, the inventor has found significant problems with respect to transfer of substrates onto and from conventional substrate holders. In dropping substrates onto the holders, substrate slide makes it difficult to accurately place the substrate in the center of the holder with good reproducibility. When the holder is heated, particularly when the holder is a heated susceptor in a cold-wall reactor, a substrate dropped onto the susceptor also tends to curl due to transitory temperature differentials within the substrate. Curl can cause “jump” and move the substrate from its desired position. Due to unpredictable placement of the substrate upon the susceptor, it is difficult to accurately tune the temperature control mechanisms for uniform heating of the substrate, especially for low temperature processes.
Furthermore, curl can cause scratching of the wafer-handling end effector and dropping of the substrate, leading to particulate contamination. In removing the wafer from the holder, the wafer tends to stick to the holder (known as “stiction”), sometimes lifts the susceptor and drops it back onto the supporting structure, thus causing further particle generation. These particle problems can cause contamination of whole wafers, or even batches of wafers, significantly reducing yield.
The illustrated substrate comprises a semiconductor wafer 16 with a generally circular edge 17, shown in
A central temperature sensor or thermocouple 28 extends through the shaft 24 and the spider 22 in proximity to the wafer holder 200. Additional peripheral thermocouples 30 are also shown housed within a slip ring or temperature compensation ring 32, which surrounds the wafer holder 200 and the wafer 16. The thermocouples 28, 30 are connected to a temperature controller (not shown), which sets the power of the various heating elements 14 in response to the readings of the thermocouples 28, 30.
In addition to housing the thermocouples 30, the slip ring 32 absorbs radiant heat during high temperature processing. This compensates for a tendency toward greater heat loss at the wafer edge 17, a phenomenon that is known to occur due to a greater concentration of surface area for a given volume near such edges. By minimizing edge losses and the attending radial temperature non-uniformities across the wafer 16, the slip ring 32 can help to prevent crystallographic slip and other problems associated with temperature non-uniformities across the wafer. The slip ring 32 can be suspended by any suitable means. For example, the illustrated slip ring 32 rests upon elbows 34, which depend from the quartz chamber dividers 36.
With reference to
A bottom plan view of the susceptor is shown in FIG. 3B. The bottom surface 210 includes a bottom groove 214, about 0.250 inch in width and with a rectangular cross section, which forms a nearly complete concentric circle and has an outer diameter of about 5.250±0.005 inches. The bottom groove 214 does not form a complete circle but is interrupted by a section 216, shown on the right side of FIG. 3B. The illustrated interrupting section 216 has a length of about 0.250 inch. The bottom groove 214 accepts fingers of the spider 22 (
The difference in height between the protrusion top 228 and the grid groove bottom 224 is preferably between about 0.35 mm and 0.55 mm, more preferably between about 0.40 mm and 0.45 mm (nominal 0.43 mm or 0.017 inch in the illustrated embodiment). The pitch of the grid, or distance between identical adjacent features, is preferably between about 1.0 mm and 1.5 mm, more preferably between about 1.2 mm and 1.3 mm in both directions (nominally 1.27 mm or 0.050 inch in the illustrated embodiment).
Similar gridded susceptors have been available from ASM America, Inc. of Phoenix, Ariz. for use in the Epsilon™ series of CVD reactors. These susceptors, however, had different grid configurations. For instance, the pitch of the grid in prior susceptors was about half that of the preferred embodiment. Furthermore, the prior susceptors were designed to be nearly planar, with the exception of a minimal manufacturing tolerance for concavity (e.g., from 0 to 0.005 inch or 0.127 mm), as compared to the peripheral portions of the grid, simply to avoid a convex shape. As detailed below with respect to
With reference to
The concavity of the upper surface 229 is defined by the difference 246 in depth, relative to the top of the shoulder 206, for example, between the top of a grid protrusion 242 on the periphery (adjacent to the annular groove 204) and the top of the grid protrusion 244 (at the center 240 of the pocket 202). The concavity 246 is preferably between about 0.130 mm and 0.500 mm. For susceptors designed for 150-mm wafers, the concavity 246 is more preferably between about 0.130 mm and 0.200 mm, and most preferably between about 0.130 mm and 0.150 mm (nominally 0.1397 mm or 0.0055 inch in an exemplary 150-mm embodiment). For susceptors designed for 200-mm wafers, the concavity 246 is more preferably between about 0.130 mm and 0.250 mm, and most preferably between about 0.170 mm and 0.220 mm (nominally 0.1905 mm or 0.0075 inch in an exemplary 200-mm embodiment). For susceptors designed for 300-mm wafers, the concavity 246 is more preferably between about 0.300 mm and 0.500 mm, and most preferably between about 0.360 mm and 0.400 mm. As the substrate size increases, the concavity is preferably increased more than proportionately.
The susceptor's enclosed flow volume and total escape area are optimized to minimize wafer slide, stick and curl while maintaining good thermal exchange properties and minimizing the risk of process gases reaching the backside of the wafer. In preferred embodiments, these parameters have been separately tailored for wafers with 6-inch or 150-mm diameters, with 8-inch or 200-mm diameters and with 12-inch or 300-mm diameters. For larger wafer sizes, the flow volume and total escape area preferably increase more than proportionately to the wafer areas because of the increased distance and increased volume of gas traveling to reach the wafer periphery.
As noted, the total enclosed flow volume includes the volume between the substrate lower surface 249 and the susceptor pocket upper surface 229 defined by the protrusion tops 228, as well as the volume within the grid grooves 222. For a susceptor designed to support a 150-mm wafer, the total enclosed flow volume is preferably between about 7.5×10−6 m3 and 10.0×10−6 m3 and more preferably between about 8.0×10−6 m3 and 9.0×10−6 m3. For susceptors designed to support wafers 200 mm in diameter or larger, the total enclosed flow volume is between 1.3×10−5 m3 and 6.0×10−5 m3. The total enclosed volume for a susceptor designed to support a 200-mm wafer is preferably between about 1.3×10−5 m3 and 4.0×10−5 m3, more preferably between about 1.4×10−5 m3 and 1.6×10−5 m3 (1.506×10−5 m3 in an exemplary 200-mm susceptor). For a susceptor designed to support a 300-mm wafer, the total enclosed flow volume is preferably between about 3.0×10−5 m3 and 6.0×10−5 m3, more preferably between about 3.5×10−5 m3 and 4.5×10−5 m3 (or 4.062×10−5 m3 in an exemplary 300-mm susceptor).
For 6-inch or 150-mm wafers, the escape area of the susceptor is preferably between about 0.1×10−4 m2 and 5.0×10−4 m2 and more preferably between about 2.0×10−4 m2 and 3.0×10−4 m2. For 200-mm wafers, the escape area of the susceptor is preferably between about 2.0×10−4 m2 and 4.0×10−4 m2 and more preferably between about 2.5×10−4 m2 and 3.5×10 m2 (3.096×10−4 m2 in an exemplary 200-mm susceptor). In another embodiment of the current invention, these parameters are optimized for 12-inch or 300-mm wafers. The escape area is preferably between about 3.0×10−4 m2 and 6.0×10−4 m2, more preferably between about 4.0×10−4 m2 and 5.0×10−4 m2 (4.643×10−4 m2 in an exemplary 300-mm susceptor).
In the preferred embodiment, a susceptor with a flat pocket surface is machined from graphite. The susceptor is coated with silicon carbide at an elevated temperature, preferably between about 1000 C and 1500 C, more preferably between 1250 C and 1350 C. As the susceptor cools down the mismatch in thermal expansion coefficient between the graphite and the silicon carbide cause the susceptor to bend, causing the concavity of the pocket surface. This embodiment is meant to illustrate a preferred method of introducing concavity into the susceptor pocket surface. The skilled artisan will find other methods for producing concavities according to the preferred embodiments described above.
The introduction of concavity into the susceptor surface, along with deeper and wider grooves, produces the total enclosed flow volume. This flow volume, along with the escape area (total cross-sectional area of the groove openings at the wafer perimeter), can accomodate the sudden drop of a wafer without creating a compressed cushion of gas with enough pressure to cause the wafer to skate or slide. In part, the enlarged flow volume assists in absorbing the compression of gas without excessive pressurization and, in part, the escape area enables the gas to escape quickly so that not enough of a cushion is created upon drop-off to even temporarily support the wafer. The wafer rests upon the support grid in the centered location for which the wafer transfer robot was programmed.
In addition, using the pocket profiles of the preferred embodiments, no significant suction effect is created between the wafer and the susceptor when the wafer is picked up by a robot end effector, such as a Bernoulli wand. This improvement in pick-up reduces particle generation.
Advantageously, the preferred flow volumes and escape areas are achieved without creating a sharp thermal transition, such as a ledge, in the susceptor. Thus, the grid continues to extend beneath the entire wafer surface, and the small space between the central protrusions and the wafer has no adverse effect on the thermal interaction between the wafer and the susceptor. In fact, improvements in film deposition uniformity and repeatability are achieved due to more consistent placement of the wafer with respect to the susceptor, such that temperature tuning applies equally well to multiple wafers sequentially processed. In contrast, prior susceptor designs showed localized cold spots when the wafer slid into contact with the pocket edge. Repeated tests of depositions performed on a wafer supported by a gridded susceptor in accordance with the preferred embodiments showed excellent reproducibility and low standard deviation in deposited layer thickness. Table I illustrates the deposition uniformity achieved with the new susceptors.
TABLE I
Mean Film
Wafer
Thickness (Å)
Standard Deviation (Å)
1
1101
5.5
2
1101
7.0
3
1099
7.2
4
1108
8.2
5
1098
7.2
6
1095
7.2
7
1098
7.2
8
1094
6.4
9
1090
7.3
10
1095
6.4
11
1100
6.9
12
1094
7.1
13
1088
6.6
14
1098
7.8
15
1103
6.0
16
1100
7.1
17
1115
8.2
18
1119
8.9
19
1115
9.8
20
1105
8.9
21
1112
7.5
22
1111
8.6
23
1113
9.8
Moreover, better control over centered drop-off also avoided catastrophic failure in processing. In depositions performed using the preferred embodiments, no slide was observed; wafers remained centered as dropped, and the deposited films were within control limits.
In experiments performed using a gridded susceptor without concavity and with narrower grooves, on the other hand, unacceptable films were observed, as indicated in Table II. The wafers in Tables I and II were processed in the same type of chamber and in a similar temperature range, although the process recipes were not identical.
TABLE II
Mean Film
Wafer
Thickness (Å)
Standard Deviation (Å)
1
1703
12.2
2
1706
8.4
3
1724
10.1
4
1662
48.6
5
1709
7.9
6
1706
12.3
7
1635
45.0
8
1635
42.8
9
1709
8.7
10
1709
11.4
11
1709
8.2
12
1705
11.3
13
1720
7.2
14
1706
8.7
15
1611
53.2
16
1635
44.8
17
1709
7.5
18
1701
13.5
19
1711
15.0
20
1705
16.6
21
1697
16.3
22
1709
10.1
Wafers 4, 7, 8, 16 and 17, had radically higher standard deviations and therefore, much greater variation in film thickness as compared to the wafers in Table I. This is an indication that wafer slide to contact the shoulder 206 of the susceptor 200 has occurred for these wafers. A significant temperature differential due to this contact results in varying deposition rates across the wafer 16 and, therefore, a non-uniform film thickness.
Additionally, the concavity 246 (
In contrast, the susceptor concavity in the preferred embodiments makes a susceptor that conforms better to the shape of the slightly curled wafer when the wafer is dropped off, thus making more continuous thermal contact therewith. Wafer curl is reduced to a concavity of about 0.200 inch, which is insufficient to scratch the Bernoulli wand. By avoiding wand scratching and attendant particle problems, yield has been shown to improve by 30% to 40%.
The susceptor 200 includes a central gridded pocket 202, on which a wafer can be positioned, and a plurality of centering locators 250 or 252 on the periphery of the pocket 202. The locators 250 or 252 extend above the level of the grid and keep the wafer from coming into contact with the annular shoulder 206, which would cause cooling at the wafer edge, as mentioned above. The wafer edge can make contact with the locators 250 or 252, so the locators 250 or 252 are designed to minimize any adverse thermal impact this might have. The centering locator 250 of
The width of each locator along a circumference of the recess, or along surfaces 260, 262 in
The locators 250 and 252 are positioned with their inner edges 260 and 262, respectively, along a circle with a diameter slightly larger than the diameter of the wafer. Preferably, there are between 6 and 10 locators, more preferably between about 7 and 9 locators, and most preferably about 8 locators spaced evenly around the edge of the pocket 202, as shown in FIG. 8.
The substrate support described herein contains features that make improvements over the prior art for many aspects of substrate positioning in substrate processing systems. In particular, the enclosed flow volume in the susceptor pocket under the substrate and the escape area around the periphery of the substrate have been designed to reduce the possibility of substrate slide and stick during drop off and pick up. This ensures that the substrate is positioned near the center of the susceptor pocket during processing. In another embodiment, centering locators around the edge of the susceptor pocket reduce further the possibility of contact between the substrate and the outer edge of the susceptor. These features of the susceptor pocket prevent contact between the edge of the substrate and the cooler outer shoulder of the substrate holder, thereby helping to achieve temperature uniformity and ensuring that the quality of the process result has good repeatibility.
It will be appreciated by those skilled in the art that various modifications and changes may be made without departing from the scope of the invention, and all such modifications and changes are intended to fall within the scope of the invention, as defined by the appended claims.
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